Tissue characterization by its electromagnetic reflective properties, for differentiating between tissue types, is known. In general it involves the propagation of an electromagnetic wave at about the microwave range, in a coaxial cable, from an electromagnetic-wave generator to the tissue to be characterized. At the proximal end with respect to the tissue, the coaxial cable may be cut and brought in contact with the tissue. Alternatively, various geometries may be provided, as coaxial endings, operative as a tissue probes.
For example Burdette, et al. [Burdette et al, “In Vivo Probe Measurement Technique for Determining Dielectric Properties at VFW Through Microwave Frequencies”, IEEE Trans. On Microwave Theory & Techniques, MTT-28 (4): 414-427, 1980] describe theoretically and experimentally the use of a probe technique in order to determine the dielectric properties of semisolid material and living tissue, in situ. This method is advantageous compared to previous methods known by the following:
1. enabling measurements of the dielectric properties in living tissue in a continuous frequency range of between about 0.1 GHz and about 10 GHz,
2. eliminating the need for tedious sample preparation, and
3. enabling data processing on a real-time basis.
The Burdette idea is to use a short monopole antenna, suitable for insertion into living tissues, as the in vivo probe. The probe is designed as a coaxial cable having an outer and an inner (center) conductor separated by a Teflon dielectric material. The inner conductor cable is slightly longer than the outer one in order to create an electric field of a monopole at the distal tip with respect to operator. This tip is to be inserted into the tissue, which dielectric properties are to be measured. The outer conductor may be grounded for minimizing fringe effects. An SMA connector is attached to the probe by first removing the inner conductor and the Teflon dielectric material, soldering it to the outer conductor and then reassembling the probe with the center conductor as the center pin of the connector. While disassembled, the probe conductors are flashed with nickel plating and then plated with gold in order to reduce chemical reactions between the probe and the electrolyte within the tissue to be examined. This process virtually eliminates oxidation of the probes metallic surfaces and helps minimize electrode polarization effects at lower frequencies.
U.S. Pat. No. 5,744,971, to Chan et al., teaches the use of a coaxial probe for measuring the dielectric properties of materials suitable, although not exclusively so, for the use in the non-invasive monitoring of the conservation treatment of cultural material e.g. works of art such as canvas. The probe is a needle like device with the coaxial structure extending to the distal tip with respect to the operator. The probe is extracorporeal as opposed to the invasive probe of Burdette. The design of this coaxial probe differs slightly from the one of Burdette et al.
U.S. Pat. No. 6,026,323, to Skladnev et al. describes a probe to characterize tissue types that combines optical and electrical tests in a single device, capable of providing the optical and electrical data almost simultaneously from very small areas of a tissue surface. Key to this approach is an instrument capable of making almost simultaneous electrical and optical measurements on the same small areas of tissue. Each measurement involves a complex sequence of events which includes: optical and electrical tissue stimulations with subsequent detection, filtering and digitization of the tissue response; extraction of specific parameters from the optical and electrical signals; checking for errors, and subsequent classification of the extracted parameters into various tissue type categories; and feedback to the system operator. The probe has a central optical fiber, which conducts electromagnetic radiation to a photo-detector diode in the handle and is positioned in the center of a bundle of optical fibers all of which are located within an external tube. A three gold electrodes are positioned adjacent and abutting against the internal surface of the external tube. The probe cable consists of many individual coaxial conductors with a single overall braided shield, enclosed in a medically rated silicone outer jacket. Both ends of the cable have round plastic pin male connectors. The electrodes and optical fibers come into direct contact with the tissue for stimulation and detection of the tissue characteristics. The probe tip is polished and smoothed and has contoured edges. An epoxy resin electrically insulates and seals the tip section.
Commonly owned U.S. Pat. No. 6,813,515 to Hashimshony teaches a probe, method and system for examining tissue, in order to differentiate it from other tissue, according to its dielectric properties. The method is of generating an electrical fringe field in the examined tissue to produce a reflected pulse therefrom with negligible radiation penetrating into the tissue itself; detecting the reflected electrical pulse; and comparing electrical characteristics of the reflected electrical pulse with respect to the applied electrical pulse to provide an indication of the dielectric properties of the examined tissue. The measuring device is built as a coaxial probe with cavity at its distal tip with respect to operator where a sample of the tissue to be examined is confined. The probe itself has an inner conductor insulated from, and enclosed by, an outer conductor open at one end and extending past the inner conductor in the axial direction, defining an open cavity at the distal end of the probe with respect to the operator. The inner conductor includes a tip within the open cavity, which tip is formed with at least two different diameters for enhancing the electrical fringe field.
U.S. Pat. No. 6,370,426, to Campbel et al., describes a method and apparatus for measuring relative hydration of a substrate. Measurements of the electrical characteristics of the substrate, the force applied to it, and the temperature of the substrate during the measurement provide inputs for determining such relative hydration of the substrate. The structure of the sensor used in this case is of two coaxial conductors one of which runs along the axis of symmetry, separated by a coaxial insulator and having a coaxial insulator outside the outer conductor. Both conductors and the separating insulator end at a plane perpendicular to the axis of symmetry at the distal tip with respect to the operator, so that the coaxial structure comes to contact with the examined tissue but does not penetrate it.
British Patent GB01153980, to Einat et al., describes an RF antenna, operative as a probe for near field identification and characterization. It has first and second radiative portions, generating electromagnetic fields, which are substantially opposing, so as to suppress far field radiation. The far-field suppression minimizes contribution from the far field, when near field characterization is sought.
U.S. Pat. No. 6,380,747, to Goldfine, et al., describes a method for processing, optimization, calibration, and display of measured dielectrometry signals. A property estimator is coupled by way of instrumentation to an electrode structure and translates sensed electromagnetic responses into estimates of one or more preselected properties or dimensions of the material, such as dielectric permittivity and ohmic conductivity, layer thickness, or other physical properties that affect dielectric properties, or presence of other lossy dielectric or metallic objects. A dielectrometry sensor is disclosed which can be connected in various ways to have different effective penetration depths of electric fields but with all configurations having the same air-gap, fluid gap, or shim lift-off height, thereby greatly improving the performance of the property estimators by decreasing the number of unknowns. The sensor geometry consists of a periodic structure with, at any one time, a single sensing element that provides for multiple wavelength within the same sensor footprint.
The systems described hereinabove are non-resonating, so the differences between signals from different tissue types are small.
By contrast, U.S. Pat. No. 5,227,730, to King, et al., U.S. Pat. No. 5,334,941, to King, and U.S. Pat. No. 6,411,103, to Tobias add an element of resonance.
U.S. Pat. No. 5,227,730, to King, et al. teaches a method and apparatus for sensing complex dielectric properties of lossy (dissipative) dielectric materials in vivo or in vitro, particularly biological tissue. This idea is based on a needle-like resonant sensor, which is inserted into the test material for measuring its dielectric properties at the resonant frequency. The major advantage, compared to the sensors described hereinabove, is that due to the resonating effect, the dielectric constants can be measured with a greater accuracy and resolution, and over a much larger volume (of the order of a cubic centimeter). Thus, the resonant sensor is able to better distinguish between tumors and normal tissue. The needle-like resonant sensor, as designed by King, et al., has the form of a dipole resonator that is positioned parallel and adjacent to a miniature coaxial feed cable and is electrically insulated from it. The dipole resonator is inductively coupled to the microwave power in the coaxial cable by means of an electrically short circumferential gap cut in the cable shield. By coupling the gap to the dipole at its center currents are induced in the dipole in a perfectly balanced and symmetric manner. With proper design of the feed gap, the dipole impedance can be well matched to the coaxial cable with very small reflection from the gap at the resonant frequency of the dipole. To regulate the degree of coupling between the dipole and the test medium, a thin cylindrical dielectric sheath encloses the entire assembly. Such a sheath might be, for example, a dielectric catheter into which the coaxial cable with its attached dipole resonator is inserted.
U.S. Pat. No. 5,334,941, to King, describes a highly sensitive, direct-contact, in situ sensor for nondestructively measuring or monitoring the complex dielectric and conductive properties of solids, liquids, or gasses at microwave frequencies. A metal microstrip dipole resonator is etched on the surface of a dielectric substrate which is bonded to a copper ground plane. The dipole resonator is electromagnetically driven by mutual inductive coupling to a short nonresonant feed slot formed in the ground plane. The slot is driven by a coaxial feed line or a microstrip feed line extending from a swept microwave frequency source which excites the incident wave. Alternatively, the metal resonator is omitted and the length of the slot is increased so that it becomes the resonator. In use, the sensor is placed in close physical contact with the test material having complex dielectric constant .epsilon.* (=.epsilon.′-j.epsilon.″) or conductivity .sigma. As the frequency of the microwave source is swept, a sharp dip in the reflected wave occurs at the resonant frequency, provided that the coaxial feed line or microstrip feed line is nearly critically coupled to the sensor input. Measurement of the resonant frequency and input coupling factor determines small changes in .epsilon.′, .epsilon.″ and .sigma. with great resolution. To diminish the electromagnetic coupling between the resonator and the test material, and to protect the resonator from damage and wear, a superstrate may be added.
U.S. Pat. No. 6,411,103, to Tobias, et al., describes a stray-field sensor for measuring dielectric properties of substances includes generating elements for generating an electrical field and shielding elements for shielding the generated electrical field. The shielding elements have at least two openings for coupling the electrical field out into the outside space so that the electrical field is at least partially located outside of the shielding elements.
Additionally, German applications DE 19705260A1 DE 19734978A1 describe systems in which the substances to be examined are brought into the resonator, to influence the resonant frequency of the resonant circuit.